1. Field of the Invention
The present invention relates generally to a fluid-filled resilient bushing assembly adapted to operate utilizing elasticity or resiliency of an elastometic or rubber material, and a resistance of an orifice to a flow of a fluid therethrough. More particularly, the invention is concerned with an improvement in the structure defining an orifice formed in a fluid-filled, bushing-type mounting assembly for vibration damping and isolation.
2. Discussion of the Related Art
In the recent years, a fluid-filled resilient bushing which utilizes elasticity of a rubber material and a fluid flow resistance has been found useful as a vibration damping and isolating mounting structure capable of providing a large degree of vibration damping effect through the fluid flow resistance, for example, as a suspension bushing or engine mount for automotive vehicles. A typical example of such a bushing mounting structure is disclosed in Publication Nos. 48-36151 and 52-16554 of Japanese Patent Applications, which were published for opposition purpose in 1973 and 1977. In the disclosed bushing structure, an inner sleeve and an outer sleeve are disclosed in coaxial and radially spaced-apart relation with each other, and a generally annular resilient member is interposed between the inner and outer sleeves. The mounting structure has a plurality of fluid chambers corresponding to a corresponding number of voids or pockets formed in the resilient member. The fluid chambers are spaced apart from each other in the circumferential direction of the bushing structure. The fluid chambers are filled with a suitable incompressible fluid, and communicate with each other through an orifice or other suitable passage means, so that the fluid may flow from one chamber to another.
In the fluid-filled resilient bushing mounting structure disclosed in the above-identified prior art documents, the construction for defining the orifice for fluid communication between the fluid chambers consists of an inner pipe which is inserted through the resilient member. Another type of construction for defining such an orifice is proposed in Laid-Open Publication No. 56-164242 of Japanese Patent Application (laid open in 1981), wherein an orifice is defined between a sleeve secured to the outer surface of a resilient member, and an outer sleeve press-fitted on that sleeve. A further modified type of construction for defining an orifice is used in a fluid-filled resilient bushing disclosed in U.S. Pat. No. 4,588,174 (claiming the priority of Japanese Patent Aplication No. 58-398l8 laid open as Publication No. 59-164428) the co-owner of which includes the assignee of the present application. In the bushing disclosed in this document, a radial stopper member is fitted on the outer surface of an inner sleeve, such that the stopper and the inner sleeve cooperate to define an orifice through which a plurality of fluid chambers or pockets are held in fluid communiction with each other.
In the fluid-filled bushing structures referred to above, the maximum phase angle frequency "c" at which the vibration damping effect attained by means of the resistance of the orifice to a fluid flow, is determined by a cross sectional area of the orifice divided by a length of the orifice. Thus, the maximum phase angle frequency decreases with a decrease in the cross sectional area of the orifice and with an increase in the length of the orifice. However, if the orifice has an excessively small cross sectional area, the amount of flow of the fluid through the orifice is not enough to provide a sufficient damping effect. For the orifice to provide a high degree of damping for low frequency vibrations, that is, to have the maximum phase angle frequency within a low frequency band of vibrations which are required to be damped, it is effective to form the orifice with a relatively large cross sectional area for permitting a sufficient flow of the fluid therethrough, and with a length as large as possible.
In the prior art fluid-filled resilient bushing structures discussed above, however, it is impossible to form the orifice having a length larger than about a half of the full circumference of the inner sleeve or outer sleeve. Therefore, the cross sectional area must be made relatively small to adapt the maximum phase angle frequency of the bushing structure to the desired low frequency band of vibrations. This will cause an inconvenience that the flow of the fluid through the orifice tends to be small as indicated above, and the damping effect for the low frequency vibrations is accordingly reduced.